Neither int: nor Special:ExpandedTemplates is a reliable way to check how the message will work in action, especially for JavaScript messages.

However, if we assume that parsing will be okay and you just want to see the different versions, then I recommend Special:ExpandTemplates. Remember to set title to the full message title, so that correct language will be used for plurals, etc. and then use it like a template: {{MediaWiki:Notification-header-mention-status-bundle/fr|param1|param2..}}.

The downside is of course that you need to know and give all the parameters.

However, I don’t achieve to give params. In the follwing code, params are not taken into account:
{{MediaWiki:Notification-header-mention-status-bundle/fr|aaa|Pols12|5|User talk:Pols12|3|2}}.

The following code work: {{int:Notification-header-mention-status-bundle/fr|Bastard|Pols12|5|User talk:Pols12|3|2}}
But, as I explained to Nemo, int: display the real message, and I’d like to test a new translation, not waiting for their daily export.